1. Field of the Invention
The present invention relates to a high-frequency filter used in a microwave band and a millimeter-wave band.
2. Description of the Related Art
With a bandstop filter described in a document entitled “Exact Design of Band-stop Microwave Filters” (written by B. M. Schiffman and G. L. Matthaei in IEEE Trans. on MTT, vol. MTT-12, pp 6-15 (1964)), for instance, by reflecting a signal in a frequency band in which the electrical length of an inner conductor of a resonator becomes approximately 90 degrees, passage of the signal in the frequency band is inhibited.
In the case of this bandstop filter, a frequency, at which the resonator resonates, becomes the center frequency of a stop band. Also, a gap of a portion, in which the inner conductor of the resonator and an inner conductor of a main line are arranged parallel to each other and constitute a line joint, corresponds to the stop bandwidth of the filter. That is, there is a property with which it is possible to enlarge the stop bandwidth by enlarging the joint between the resonator and the main line through reduction of the gap of the line joint portion.
Further, the joint between the resonator and the main line described above becomes the maximum when the electrical length in the line joint portion at the center frequency of the stop band is 90 degrees. That is, when it is desired to secure a predetermined joint amount between the main line and the resonator in the case where the electrical length in the line joint portion at the center frequency of the stop band is smaller than 90 degrees, it is required to reduce the gap of the line joint portion likewise.
However, the conventional technique has the following problems. The size of the gap of the line joint portion described above depends on the kind of the line constituting the filter. In addition, because of the producible minimum size, production errors, and the like, it is not guaranteed that the size of the gap necessarily becomes a desired size. This imposes a limitation on the stop bandwidth that is realizable with a produced filter.
In particular, when the conventional bandstop filter is constructed using a planar circuit such as a microstrip line or a strip line, there arise the following problems. That is, a strip conductor corresponding to the inner conductor described above has an extremely thin thickness, which makes it more difficult to obtain a large joint. When a gap for realizing a desired stop bandwidth is reduced and approaches a limitation in terms of production, a problem of variation in gap due to a production error or variation in width due to a production error of two strip conductors becomes more prominent. As a result, variation in characteristics due to the variation leads to variation in stop band frequency. However, it is difficult to adjust the distance between the strip conductors after formation because they are formed through etching or the like. Therefore, the variation in characteristics due to the production error directly leads to a filter yield reduction.
In addition, the conventional bandstop filter has a problem in that a production error in short-circuiting means of the resonator directly leads to variation in filter characteristics. In particular, when the filter is constructed using a planar circuit such as a microstrip line, the short-circuiting means is formed using a through hole or a via hole. In such a case, there is a problem in that when the positional relation between the strip conductor and the through hole (via hole) changes due to a problem in terms of production, a resonance frequency is shifted and there occurs characteristic deterioration such as variation in stop band.